An
astrolabe consists of a circle marked in degrees (similar to a protractor) with
a rotating arm attached at its center. When the 0° mark on the circle is aligned
with the horizon, and a star (or other celestial body) "sighted" at the end of
the movable arm, the position (in degrees) of the star can be read ("taken") off
the calibrated circle (hence, "astro" = star + "labe" = to take).

On
the plate (mater) are engraved coordinate lines which represent a stereographic projection
of the celestial sphere (climate),
valid for places at a specific geographic latitude. Over this coordinate grid
rotates the rete, which is a framework with spikes whose points represent
fixed stars. After adjusting the instrument for the current time, the position
of a star can be read from the coordinate grid. Conversely, the instrument can
be adjusted to fit the measured position, and the time can be read off the scale.

In
the 15th century, the French instrument-maker Jean
Fusoris (ca. 1365 - 1436) started selling astrolabes in his shop in Paris,
along with portable sundials and other popular scientific gadgets of the day.